Electronic device display with a backlight having light-emitting diodes and driver integrated circuits in an active area

ABSTRACT

A pixel array may be illuminated with backlight illumination from a backlight. The backlight may include a two-dimensional array of light-emitting diodes, with each light-emitting diode being placed in a respective cell. Different light-emitting diodes may have unique brightness magnitudes based on the content of the given display frame. Driver integrated circuits may control one or more associated light-emitting diodes to have a desired brightness level. The driver integrated circuits may be formed in an active area of the backlight. The driver integrated circuits may be arranged in groups that are daisy chained together. A digital signal (that includes information such as addressing information) may be propagated through the group of driver integrated circuits. To manage thermal performance of the backlight, the backlight may include a thermally conductive layer and/or a heat sink structure. To increase the efficiency of the backlight, the backlight may include one or more reflective layers.

This application claims the benefit of provisional patent applicationNo. 63/029,048, filed May 22, 2020, provisional patent application No.63/029,069, filed May 22, 2020, and provisional patent application No.63/029,082, filed May 22, 2020, which are hereby incorporated byreference herein in their entireties.

BACKGROUND

This relates generally to electronic devices with displays, and, moreparticularly, to displays with backlights.

Electronic devices such as computers and cellular telephones havedisplays. Some displays such organic light-emitting diode displays havearrays of pixels that generate light. In displays of this type,backlighting is not necessary because the pixels themselves producelight. Other displays contain passive pixels that can alter the amountof light that is transmitted through the display to display informationfor a user. Passive pixels do not produce light themselves, so it isoften desirable to provide backlight for a display with passive pixels.Passive pixels may be formed from a layer of liquid crystal materialformed between two electrode layers and two polarizer layers.

In a typical backlight assembly for a display, an edge-lit light guideplate is used to distribute backlight generated by a light source suchas a light-emitting diode light source. A reflector may be formed underthe light guide plate to improve backlight efficiency.

Conventional backlight assemblies may cause visible artifacts, may notbe robust, and may occupy an undesirably large amount of space within anelectronic device.

It would therefore be desirable to be able to provide displays withimproved backlights.

SUMMARY

A display may have an array of pixels for displaying images for aviewer. The array of pixels may be liquid crystal pixels formed fromdisplay layers such as a color filter layer, a liquid crystal layer, athin-film transistor layer, an upper polarizer layer, and a lowerpolarizer layer.

The pixel array may be illuminated with backlight illumination from abacklight unit. The backlight unit may include an array oflight-emitting diodes, with each light-emitting diode being placed in arespective cell. The brightness of each light-emitting diode may bechanged in each display frame to optimize the viewing of the display.Different light-emitting diodes may have unique brightness magnitudesbased on the content of the given display frame.

Driver integrated circuits may be used to control the light-emittingdiodes of the backlight. Each driver integrated circuit may control oneor more associated light-emitting diodes to have a desired brightnesslevel. The driver integrated circuits may be formed in an active area ofthe backlight. For example, the light-emitting diodes may be mounted tothe upper surface of a glass substrate. The driver integrated circuitsmay also be mounted to the upper surface of the glass substrate. Thedriver integrated circuits may be interspersed amongst thelight-emitting diodes.

The driver integrated circuits may be arranged in groups that are daisychained together. A digital signal (that includes information such asaddressing information) may be propagated through the group of driverintegrated circuits. Each driver integrated circuit may have a smallnumber of input-output contacts (pins) for minimal complexity. Thedriver integrated circuits may have four pins, six pins, or nine pins,as examples.

To manage thermal performance of the backlight, the backlight mayinclude a thermally conductive layer that is attached to a lower surfaceof the glass substrate for the light-emitting diodes. The glasssubstrate may also have exposed conductive layers that are coupled toheat sinks for additional heat dispersion. Sensors such as temperaturesensors and/or optical sensors may be formed on the upper surface of theglass substrate. The sensors may provide real-time measurements to acontroller such as a timing controller. The timing controller may, inturn, control operation of the light-emitting diodes in the backlightbased at least partially on the sensor information.

To increase the efficiency of the backlight, the glass substrate may beformed from white diffusive glass. Additionally, a reflective layer maybe formed on the upper surface of the glass substrate. A reflectivelayer may also be formed on a lower surface of the glass substrate.Reflective layers may be formed on the top surfaces of the driverintegrated circuits to prevent a shadow from appearing in the activearea of the display where the driver integrated circuits are present.The light-emitting diodes may be arranged in a non-square-grid layout toreduce periodicity and prevent mura.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative electronic devicehaving a display in accordance with an embodiment.

FIG. 2 is a top view of an illustrative display in accordance with anembodiment.

FIG. 3 is a cross-sectional side view of an illustrative display in anelectronic device that has a backlight and a pixel array in accordancewith an embodiment.

FIG. 4 is a top view of an illustrative backlight having light-emittingdiodes arranged in respective cells in accordance with an embodiment.

FIG. 5 is a top view of an illustrative display showing how differentportions of the display may have different target brightness levels inaccordance with an embodiment.

FIG. 6 is a cross-sectional side view of an illustrative backlighthaving light-emitting diodes (LEDs) on an upper surface of a substrateand LED driver integrated circuits (ICs) on the upper surface of thesubstrate in an inactive area of the display in accordance with anembodiment.

FIG. 7 is a cross-sectional side view of an illustrative backlighthaving light-emitting diodes (LEDs) on an upper surface of a substrateand LED driver integrated circuits (ICs) on a lower surface of thesubstrate in an active area of the display in accordance with anembodiment.

FIG. 8 is a cross-sectional side view of an illustrative backlighthaving light-emitting diodes (LEDs) on an upper surface of a substrateand LED driver integrated circuits (ICs) on the upper surface of thesubstrate in an active area of the display in accordance with anembodiment.

FIG. 9 is a top view of an illustrative LED array that includes driverICs distributed throughout the active area of the display in accordancewith an embodiment.

FIG. 10 is a schematic diagram of an illustrative display with a timingcontroller that provides signals directly to LED driver ICs in theactive area in accordance with an embodiment.

FIG. 11 is a schematic diagram of an illustrative display with a timingcontroller that provides signals to a backlight controller that thenprovides signals directly to LED driver ICs in the active area inaccordance with an embodiment.

FIG. 12 is a schematic diagram of an illustrative LED array with driverICs that have six pins in accordance with an embodiment.

FIG. 13 is a schematic diagram of an illustrative driver IC with ninepins for independently controlling different LED zones in accordancewith an embodiment.

FIG. 14 is a cross-sectional side view of an illustrative LED array withboth LEDs and driver ICs soldered to a glass substrate in accordancewith an embodiment.

FIG. 15 is a top view of an illustrative backlight showing howconductive layers on a glass substrate may be exposed and connected toheat sinks in accordance with an embodiment.

FIG. 16 is a cross-sectional side view of the illustrative backlight ofFIG. 15 in accordance with an embodiment.

FIG. 17 is a top view of an illustrative backlight showing howtemperature and optical sensors may be distributed across the activearea of the backlight in accordance with an embodiment.

FIG. 18 is a cross-sectional side view of an illustrative backlightshowing how a reflective layer may be attached to the lower surface of aglass LED substrate in accordance with an embodiment.

FIG. 19 is a cross-sectional side view of an illustrative backlightshowing how a reflective layer and a separate thermally conductive layermay be attached to the lower surface of a glass LED substrate inaccordance with an embodiment.

FIG. 20 is a cross-sectional side view of an illustrative backlightshowing how a reflective and thermally conductive layer may be attachedto the lower surface of a glass LED substrate in accordance with anembodiment.

FIG. 21 is a cross-sectional side view of an illustrative backlightshowing how a substrate may be formed from white diffusive glass inaccordance with an embodiment.

FIG. 22 is a cross-sectional side view of an illustrative backlightshowing how a reflective layer may be formed on an upper surface of anLED driver IC in accordance with an embodiment.

FIG. 23 is a top view of an illustrative LED array showing how the LEDsmay be arranged in a zig-zag pattern in accordance with an embodiment.

FIG. 24 is a top view of an illustrative LED array showing how the LEDsmay be arranged in a zig-zag pattern with increased zone-to-zone spacingin accordance with an embodiment.

FIG. 25 is a graph of an illustrative emission profile for LED groups ina backlight in accordance with an embodiment.

FIG. 26 is a top view of an illustrative LED array showing how a centerLED may be driven at a higher current than surrounding, peripheral LEDsto achieve a desired emission profile in accordance with an embodiment.

DETAILED DESCRIPTION

An illustrative electronic device of the type that may be provided witha display is shown in FIG. 1. Electronic device 10 may be a computingdevice such as a laptop computer, a computer monitor containing anembedded computer, a tablet computer, a cellular telephone, a mediaplayer, or other handheld or portable electronic device, a smallerdevice such as a wrist-watch device, a pendant device, a headphone orearpiece device, a device embedded in eyeglasses or other equipment wornon a user's head, or other wearable or miniature device, a display, acomputer display that contains an embedded computer, a computer displaythat does not contain an embedded computer, a gaming device, anavigation device, an embedded system such as a system in whichelectronic equipment with a display is mounted in a kiosk or automobile,or other electronic equipment. Electronic device 10 may have the shapeof a pair of eyeglasses (e.g., supporting frames), may form a housinghaving a helmet shape, or may have other configurations to help inmounting and securing the components of one or more displays on the heador near the eye of a user.

As shown in FIG. 1, electronic device 10 may include control circuitry16 for supporting the operation of device 10. Control circuitry 16 mayinclude storage such as hard disk drive storage, nonvolatile memory(e.g., flash memory or other electrically-programmable-read-only memoryconfigured to form a solid-state drive), volatile memory (e.g., staticor dynamic random-access memory), etc. Processing circuitry in controlcircuitry 16 may be used to control the operation of device 10. Theprocessing circuitry may be based on one or more microprocessors,microcontrollers, digital signal processors, baseband processors, powermanagement units, audio chips, application-specific integrated circuits,etc.

Input-output circuitry in device 10 such as input-output devices 12 maybe used to allow data to be supplied to device 10 and to allow data tobe provided from device 10 to external devices. Input-output devices 12may include buttons, joysticks, scrolling wheels, touch pads, key pads,keyboards, microphones, speakers, tone generators, vibrators, cameras,sensors, light-emitting diodes and other status indicators, data ports,etc. A user can control the operation of device 10 by supplying commandsthrough input resources of input-output devices 12 and may receivestatus information and other output from device 10 using the outputresources of input-output devices 12.

Input-output devices 12 may include one or more displays such as display14. Display 14 may be a touch screen display that includes a touchsensor for gathering touch input from a user or display 14 may beinsensitive to touch. A touch sensor for display 14 may be based on anarray of capacitive touch sensor electrodes, acoustic touch sensorstructures, resistive touch components, force-based touch sensorstructures, a light-based touch sensor, or other suitable touch sensorarrangements. A touch sensor for display 14 may be formed fromelectrodes formed on a common display substrate with the display pixelsof display 14 or may be formed from a separate touch sensor panel thatoverlaps the pixels of display 14. If desired, display 14 may beinsensitive to touch (i.e., the touch sensor may be omitted). Display 14in electronic device 10 may be a head-up display that can be viewedwithout requiring users to look away from a typical viewpoint or may bea head-mounted display that is incorporated into a device that is wornon a user's head. If desired, display 14 may also be a holographicdisplay used to display holograms.

Control circuitry 16 may be used to run software on device 10 such asoperating system code and applications. During operation of device 10,the software running on control circuitry 16 may display images ondisplay 14.

Input-output devices 12 may also include one or more sensors 13 such asforce sensors (e.g., strain gauges, capacitive force sensors, resistiveforce sensors, etc.), audio sensors such as microphones, touch and/orproximity sensors such as capacitive sensors (e.g., a two-dimensionalcapacitive touch sensor associated with a display and/or a touch sensorthat forms a button, trackpad, or other input device not associated witha display), and other sensors. In accordance with some embodiments,sensors 13 may include optical sensors such as optical sensors that emitand detect light (e.g., optical proximity sensors such astransreflective optical proximity structures), ultrasonic sensors,and/or other touch and/or proximity sensors, monochromatic and colorambient light sensors, image sensors, fingerprint sensors, temperaturesensors, proximity sensors and other sensors for measuringthree-dimensional non-contact gestures (“air gestures”), pressuresensors, sensors for detecting position, orientation, and/or motion(e.g., accelerometers, magnetic sensors such as compass sensors,gyroscopes, and/or inertial measurement units that contain some or allof these sensors), health sensors, radio-frequency sensors, depthsensors (e.g., structured light sensors and/or depth sensors based onstereo imaging devices), optical sensors such as self-mixing sensors andlight detection and ranging (lidar) sensors that gather time-of-flightmeasurements, humidity sensors, moisture sensors, gaze tracking sensors,and/or other sensors. In some arrangements, device 10 may use sensors 13and/or other input-output devices to gather user input (e.g., buttonsmay be used to gather button press input, touch sensors overlappingdisplays can be used for gathering user touch screen input, touch padsmay be used in gathering touch input, microphones may be used forgathering audio input, accelerometers may be used in monitoring when afinger contacts an input surface and may therefore be used to gatherfinger press input, etc.).

Display 14 may be a liquid crystal display or may be a display based onother types of display technology (e.g., organic light-emitting diodedisplays). Device configurations in which display 14 is a liquid crystaldisplay are sometimes described herein as an example. This is, however,merely illustrative. Any suitable type of display may be used, ifdesired. In general, display 14 may have a rectangular shape (i.e.,display 14 may have a rectangular footprint and a rectangular peripheraledge that runs around the rectangular footprint) or may have othersuitable shapes. Display 14 may be planar or may have a curved profile.

FIG. 2 is a top view of a portion of display 14 showing how display 14may have an array of pixels 22. Pixels 22 may have color filter elementsof different colors such as red color filter elements R, green colorfilter elements G, and blue color filter elements B. Pixels 22 may bearranged in rows and columns and may form active area AA of display 14.Pixels 22 may be formed form liquid crystal display layers, as oneexample. The rectangular shape of display 14 and active area AA in FIG.2 is merely illustrative. If desired, the active area AA may have anon-rectangular shape (e.g., a shape with one or more curved portions).For example, the active area may have rounded corners in one example.

A cross-sectional side view of display 14 is shown in FIG. 3. As shownin FIG. 3, display 14 may include a pixel array such as pixel array 24.Pixel array 24 may include an array of pixels such as pixels 22 of FIG.2 (e.g., an array of pixels having rows and columns of pixels). Pixelarray 24 may be formed from a liquid crystal display module (sometimesreferred to as a liquid crystal display or liquid crystal layers) orother suitable pixel array structures.

During operation of display 14, images may be displayed on pixel array24. Backlight unit 42 (which may sometimes be referred to as abacklight, backlight layers, backlight structures, a backlight module, abacklight system, etc.) may be used in producing backlight illumination44 that passes through pixel array 24. This illuminates any images onpixel array 24 for viewing by a viewer such as viewer 20 who is viewingdisplay 14 in direction 21.

Backlight unit 42 may have optical films 26, a light diffuser such aslight diffuser (light diffuser layer) 34, and light-emitting diode (LED)array 36. Light-emitting diode array 36 may contain a two-dimensionalarray of light sources such as light-emitting diodes 38 that producebacklight illumination 44. Light-emitting diodes 38 may, as an example,be arranged in rows and columns and may lie in the X-Y plane of FIG. 3.

The light produced by each light-emitting diode 38 may travel upwardlyalong dimension Z through light diffuser 34 and optical films 26 beforepassing through pixel array 24. Light diffuser 34 may containlight-scattering structures that diffuse the light from light-emittingdiode array 36 and thereby help provide uniform backlight illumination44. Optical films 26 may include films such as dichroic filter 32,phosphor layer 30, and films 28. Films 28 may include brightnessenhancement films that help to collimate light 44 and thereby enhancethe brightness of display 14 for user 20 and/or other optical films(e.g., compensation films, etc.).

Light-emitting diodes 38 may emit light of any suitable color. With oneillustrative configuration, light-emitting diodes 38 emit blue light.Dichroic filter layer 32 may be configured to pass blue light fromlight-emitting diodes 38 while reflecting light at other colors. Bluelight from light-emitting diodes 38 may be converted into white light bya photoluminescent material such as phosphor layer 30 (e.g., a layer ofwhite phosphor material or other photoluminescent material that convertsblue light into white light). If desired, other photoluminescentmaterials may be used to convert blue light to light of different colors(e.g., red light, green light, white light, etc.). For example, onelayer 30 (which may sometimes be referred to as a photoluminescent layeror color conversion layer) may include quantum dots that convert bluelight into red and green light (e.g., to produce white backlightillumination that includes, red, green, and blue components, etc.).Configurations in which light-emitting diodes 38 emit white light (e.g.,so that layer 30 may be omitted, if desired) may also be used.

In configurations in which layer 30 emits white light such as whitelight produced by phosphorescent material in layer 30, white light thatis emitted from layer 30 in the downwards (−Z) direction may bereflected back up through pixel array 24 as backlight illumination bydichroic filter layer 32 (i.e., layer 32 may help reflect backlightoutwardly away from array 36). In configurations in which layer 30includes, for example, red and green quantum dots, dichroic filter 32may be configured to reflect red and green light from the red and greenquantum dots, respectively to help reflect backlight outwardly away fromarray 36. By placing the photoluminescent material of backlight 42(e.g., the material of layer 30) above diffuser layer 34, light-emittingdiodes 38 may be configured to emit more light towards the edges of thelight-emitting diode cells (tiles) of array 36 than at the centers ofthese cells, thereby helping enhance backlight illumination uniformity.

In a configuration in which pixel array 24 is formed using a liquidcrystal display, pixel array 24 may include a liquid crystal layer sucha liquid crystal layer 52. Liquid crystal layer 52 may be sandwichedbetween display layers such as display layers 58 and 56. Layers 56 and58 may be interposed between lower polarizer layer 60 and upperpolarizer layer 54. Liquid crystal display structures of other types maybe used in forming pixel array 24, if desired.

Layers 56 and 58 may be formed from transparent substrate layers such asclear layers of glass or plastic. Layers 56 and 58 may be layers such asa thin-film transistor layer and/or a color filter layer. Conductivetraces, color filter elements, transistors, and other circuits andstructures may be formed on the substrates of layers 58 and 56 (e.g., toform a thin-film transistor layer and/or a color filter layer). Touchsensor electrodes may also be incorporated into layers such as layers 58and 56 and/or touch sensor electrodes may be formed on other substrates.

With one illustrative configuration, layer 58 may be a thin-filmtransistor layer that includes an array of pixel circuits based onthin-film transistors and associated electrodes (pixel electrodes) forapplying electric fields to liquid crystal layer 52 and therebydisplaying images on display 14. Layer 56 may be a color filter layerthat includes an array of color filter elements for providing display 14with the ability to display color images. If desired, layer 58 may be acolor filter layer and layer 56 may be a thin-film transistor layer.Configurations in which color filter elements are combined withthin-film transistor structures on a common substrate layer may also beused.

During operation of display 14 in device 10, control circuitry (e.g.,one or more integrated circuits on a printed circuit) may be used togenerate information to be displayed on display 14 (e.g., display data).The information to be displayed may be conveyed to a display driverintegrated circuit such as circuit 62A or 62B using a signal path suchas a signal path formed from conductive metal traces in a rigid orflexible printed circuit such as printed circuit 64 (as an example).Integrated circuits such as integrated circuit 62A and/or flexibleprinted circuits such as flexible printed circuit 64 may be attached tosubstrate 58 in ledge region 66 (as an example).

FIG. 4 is a top view of an illustrative light-emitting diode array forbacklight 42. As shown in FIG. 4, light-emitting diode array 36 maycontain rows and columns of light-emitting diodes 38. Eachlight-emitting diode 38 may be associated with a respective cell (tilearea) 38C. The length D of the edges of cells 38C may be 2 mm, 18 mm,1-10 mm, 1-4 mm, 10-30 mm, more than 5 mm, more than 10 mm, more than 15mm, more than 20 mm, less than 25 mm, less than 20 mm, less than 15 mm,less than 10 mm, or other suitable size. If desired, hexagonally tiledarrays and arrays with light-emitting diodes 38 that are organized inother suitable array patterns may be used. In arrays with rectangularcells, each cell may have sides of equal length (e.g., each cell mayhave a square outline in which four equal-length cell edges surround arespective light-emitting diode) or each cells may have sides ofdifferent lengths (e.g., a non-square rectangular shape). Theconfiguration of FIG. 4 in which light-emitting diode array 36 has rowsand columns of square light-emitting diode regions such as cells 38C(e.g., a two-dimensional array of cells 38C) is merely illustrative.

In some cases, each cell 38C may include a single light-emitting diode.Alternatively, each cell 38C may have a light source that is formed forman array of light-emitting diode dies (e.g., multiple individuallight-emitting diodes 38 arranged in an array such as a 2×2 group oflight-emitting diodes or 3×3 group of light-emitting diodes in each cell38C). The diodes 38 in light source 38′ may be mounted on a commonsubstrate, may be mounted on a printed circuit board substrate thatextends across array 36, may be mounted on a glass substrate thatextends across array 36, or may be mounted in array 36 using otherdesired arrangements. In general, each cell 38C may include a singlelight-emitting diode, a pair of light-emitting diodes, 2-20light-emitting diodes, at least 2 light-emitting diodes, at least 4light-emitting diodes, at least 8 light-emitting diodes, fewer than 5light-emitting diodes, between 4 and 12 light-emitting diodes, between 8and 12 light-emitting diodes, between 8 and 10 light-emitting diodes, 9light-emitting diodes, or other desired number of light-emitting diodes.

Light-emitting diodes 38 may be controlled in unison by controlcircuitry in device 10 or may be individually controlled. Controllingthe light-emitting diodes individually may enable the electronic deviceto implement a local dimming scheme that helps improve the dynamic rangeof images displayed on pixel array 24 and that potentially reduces thepower consumption of the backlight. The dynamic range of a display maybe considered the ratio between the light of the highest intensity(e.g., the brightest light) that the display is capable of emitting andthe light of the lowest intensity (e.g., the dimmest light) that thedisplay is capable of emitting.

If all of the light-emitting diodes in backlight unit 42 are controlledin unison, the dynamic range of the display may be limited. Consider theexample depicted in FIG. 5. In FIG. 5, objects such as objects 72-1 and72-2 are displayed on display 14 (sometimes referred to as screen 14).In this example, object 72-1 may have a high brightness level. Object72-2 may have an intermediate brightness level. The background of thedisplay may have a low brightness level. If the light-emitting diodesproviding backlight for display 14 in FIG. 5 are controlled in unison,all of the light-emitting diodes may be set to a brightness that isoptimized for object 72-1. In this scenario, object 72-1 may bedisplayed with its intended brightness. However, the background of thedisplay is also receiving backlight with a high brightness optimized forobject 72-1. Therefore, the background of the display may appearbrighter than desired due to display limitations such as light leakagethrough the pixels or other limitations, and the dynamic range of thedisplay is lower than desired. Alternatively, all of the light-emittingdiodes may be set to a brightness that is optimized for the backgroundof the display. In this scenario, the background may be displayed withits intended brightness. However, object 72-1 is also receivingbacklight with a low brightness optimized for the background. Therefore,object 72-1 will appear dimmer than desired and the dynamic range of thedisplay will be lower than desired. In yet another embodiment, thebrightness of all of the light-emitting diodes may be set to abrightness that is optimized for object 72-2. In this scenario, object72-1 will appear dimmer than desired and the background will appearbrighter than desired.

Additionally, controlling all of the light-emitting diodes in backlightunit 42 in unison may introduce power consumption limitations. Themaximum allowable power consumption of the backlight unit may preventall of the light-emitting diodes from being operated at a peakbrightness level. For example, all of the light-emitting diodes may notbe able to emit light with a desired brightness for bright object 72-1while meeting power consumption requirements.

To summarize, operating all of the light-emitting diodes in thebacklight in unison such that the backlight brightness is the sameacross the display forces trade-offs in the aesthetics of the displayedimage. Portions of the display may be dimmer than desired or brighterthan desired and the dynamic range of the display will be lower thandesired.

To increase the dynamic range of the display (and to allow for peakbrightness levels without exceeding power consumption requirements), thelight-emitting diodes in backlight unit 42 may be controlledindividually. For example, light emitting diodes in region 14-1 of thedisplay may have a high brightness optimized for the high brightness ofobject 72-1, light-emitting diodes in region 14-2 of the display mayhave a brightness optimized for the intermediate brightness of object72-2, and light-emitting diodes in region 14-3 of the display may have alow brightness optimized for the low brightness of the background of thedisplay. In one example, the light-emitting diodes in region 14-1 mayoperate at a maximum brightness whereas the light-emitting diodes inbackground region 14-3 may be turned off (e.g., operate at a minimumbrightness). Varying the brightness of the light-emitting diodes acrossthe display in this manner increases the dynamic range of the display.

Having a two-dimensional array of independently controllable lightsources such as light-emitting diodes 38 for producing backlightillumination 44 therefore may increase the dynamic range of the display.Backlights with two-dimensional arrays of light-emitting diodes maysometimes be referred to as two-dimensional backlights. These types ofbacklights may also sometimes be referred to as direct-lit backlights.The direct-lit backlights emit light vertically towards the pixel array,as opposed to backlights with edge-lit light guide plates (where lightis emitted parallel to the plane of the pixel array and redirectedvertically towards the pixel array by the light guide plate).

Driving circuitry may be included in display 14 to controlling thelight-emitting diodes in backlight 42. Driving circuitry for the LEDsmay be formed from integrated circuits, thin-film transistor circuits,and/or other suitable circuitry. In one example, driving circuitry maybe incorporated as thin-film transistor circuitry on a rigid printedcircuit board (e.g., a printed circuit board with a plurality of layersof dielectric material such as polyimide and conductive layers).However, the costs associated with such an arrangement may be high,particularly in backlights with a high number of light-emitting diodes.An alternative arrangement for the LED driving circuitry is for driverintegrated circuits (sometimes referred to as driver ICs) to be includedin backlight 42. Each driver IC may control one or more correspondinglight-emitting diodes. In this way, the light emitting diodes may becontrolled to have varying brightness magnitudes across the backlight.The driver integrated circuits may also be used in combination with aglass substrate in one example. In other words, instead of thelight-emitting diodes and driver ICs being mounted on a rigid printedcircuit board (e.g., formed using polyimide), the light-emitting diodesand driver ICs may be mounted on a glass substrate. The glass substratemay have conductive traces (e.g., copper traces) to allow signals to betransferred between components as necessary.

There are numerous options for how to mount the LEDs and correspondingdriver ICs on a substrate. As shown in FIG. 6, LEDs 38 are mounted on anupper surface of substrate 84. In the arrangement of FIG. 6, the driverICs 82 are also mounted on the upper surface of substrate 84 in theinactive area IA. The area of the backlight with LEDs 38 corresponds tothe active area AA of the display (e.g., the area of the display thatemits light). The backlight 42 may be described as having an active areaand inactive area, similar to the pixel array. The active area of thebacklight and the active area of the pixel array may have the samefootprint. The inactive area of the backlight and the inactive area ofthe pixel array may have the same footprint or may optionally havedifferent footprints. The active and inactive areas of either thebacklight or pixel array may sometimes simply be referred to as simplythe active and inactive areas of the display.

As shown in FIG. 6, driver IC 82 is positioned at the periphery of thesubstrate in an inactive area (IA) of the display. The driver ICs mayall be positioned at the periphery of substrate 84 and may each controla corresponding group of light-emitting diodes. This type of arrangementhas numerous limitations, however. First, it is, in general, desirableto minimize the size of the inactive area. The inactive area takes upvaluable space within the electronic device without contributing to theaesthetics of the display. Having the driver ICs in the periphery of thedisplay undesirably increases the footprint of the display withoutincreasing the light-emitting area of the display. Additionally,positioning driver ICs only in the inactive area of the display may makeit challenging to increase the number of LEDs within the backlight. Byonly having the driver ICs at the periphery of the backlight, eachdriver IC may have to control a large number of light-emitting diodes(because the peripheral driver ICs have to control both the central LEDsand peripheral LEDs).

An alternate arrangement for the backlight is shown in FIG. 7. As shown,in this arrangement the light-emitting diodes 38 are positioned on anupper surface of substrate 84 whereas the driver IC 82 is positioned ona lower surface of substrate 84. Having the driver IC positioned on thelower surface in this manner eliminates the need for the large inactivearea of FIG. 6. In other words, the active area may extend virtually tothe edge of substrate 84. However, positioning the driver ICs on thelower surface of the substrate increases the total thickness 86 of thebacklight unit. Additionally, more complex conductive routing (e.g.,with conductive vias) may be required for driver IC 82 to properlycontrol the light-emitting diodes on the opposing side of substrate 84.

FIG. 8 is a cross-sectional side view of another possible arrangementwhere the driver ICs are positioned on the upper surface of substrate 84within the active area. As shown, the driver ICs may be positionedbetween LEDs of the backlight. With this arrangement, the size of theinactive area may be minimized (because the driver ICs do not increasethe size of the inactive area). Because the driver ICs are on the uppersurface of substrate 84, the thickness and complexity of the backlightmay be mitigated. Additionally, because the driver ICs are positionedwithin the active area, each LED may have a corresponding driver IC. Thedriver ICs may therefore have a low complexity and size (because eachdriver IC only needs to control a small number of LEDs). Using lowcomplexity driver ICs reduces the number of interconnects required andallows for the size of the backlight to be scaled to larger sizes (i.e.,larger numbers of LEDs within the backlight).

Digital signals may be used to control the driver ICs 82 (sometimesreferred to as LED driver ICs 82 or backlight driver ICs 82). Usingdigital control lines for the backlight may enable larger backlights ona single substrate, may reduce the total pin count for each driver IC,may reduce the number of interconnects within the backlight, and mayincrease the magnitude of drive currents enabled by the driver ICs.

As previously discussed, substrate 84 may optionally be a rigid printedcircuit board (e.g., with a plurality of insulating layers formed from adielectric material such as polyimide). Alternatively, to reduce themanufacturing cost and complexity of the LED array, substrate 84(sometimes referred to as LED substrate 84) may formed from glass.Conductive traces (e.g., copper traces) may be deposited on the glasssubstrate to allow electrical connections between components mounted tothe glass substrate.

FIG. 9 is a top view of an illustrative light-emitting diode arrayhaving driver ICs distributed throughout the active area of the display.As shown in FIG. 9, each driver IC 82 may control a corresponding LEDgroup 102. Each LED group 102 (sometimes referred to as LED zone 102)may include one or more light-emitting diodes. The light-emitting diodesmay be connected in series between a power supply line 106 and thedriver IC. In the example of FIG. 9, each LED zone 102 includes nineLEDs that are connected in series between power supply line 106 and thedriver IC.

Herein, the term LED group (or LED zone) may be used to refer to anindependently controllable group of LEDs. For example, first and secondlight-emitting diodes that are controlled separately would be referredto as first and second unique LED groups (even though there is only oneLED per group). In contrast, nine light-emitting diodes that arecontrolled together is referred to as a single LED group. Each LED grouphas an associated LED cell, which may refer to the light-emitting areaassociated with that LED group. Because the LEDs emit light across abroad range of angles (as opposed to highly collimated light), thefootprint of a light-emitting area associated with a given LED groupwill be larger than the footprint of the LED group itself. Because eachLED group is controlled to have one brightness value, the LED cellassociated with each group may have an associated single brightnessvalue. In other words, the brightness may be, ideally, uniform acrossthe LED cell. In practice, there may be some non-uniformities across theLED cell (e.g., caused by hotspots over the LEDs). Films 26 discussed inconnection with FIG. 3 may be designed to increase uniformity of lightwithin each LED cell.

Power supply line 106 may provide a power supply voltage VLED (e.g., apositive power supply voltage) across the LED array. Each LED group mayhave a light-emitting diode with a first terminal (e.g., the anode)coupled to the power supply line. The second terminal (e.g., thecathode) of that LED is then connected to the first terminal of thesubsequent LED. This chain may continue, with each LED having a firstterminal coupled to the second terminal of the preceding LED and asecond terminal coupled to the first terminal of the subsequent LED. Inthe example of FIG. 9, each LED has an anode coupled to the cathode ofthe preceding LED and a cathode coupled to the anode of the subsequentLED. The first LED in the group has an anode coupled to the supply line106. The last LED in the group has a cathode coupled to driver IC 82.

This arrangement may be reversed if desired, with the first LED in thegroup having a cathode coupled to the supply line (e.g., a ground powersupply line), the last LED in the group having an anode coupled to thedriver IC, and the other LEDs having a cathode coupled to the anode ofthe preceding LED and an anode coupled to the cathode of the subsequentLED.

As shown in FIG. 9, the driver ICs may optionally be arranged in anarray of rows and columns. Each row and column of driver ICs may includeany desired number of driver ICs. Each driver IC 82 has input-outputcontacts referred to as pins. The pins are used by the driver IC totransmit and receive signals.

In FIG. 9, each driver IC has four pins (P1, P2, P3, and P4). Varioussubsets of the driver ICs may be chained together in series (e.g.,daisy-chained). In FIG. 9, each column of driver ICs are chainedtogether. However, it should be noted that smaller larger groups ofdriver ICs may be chained together if desired.

Pin P4 may sometimes be referred to as an input pin and pin P1 maysometimes be referred to as an output pin. Pin P4 for one of the driverICs in a given column (e.g., driver IC 82-1 in FIG. 9) may receive aninput from control line 108-1. The input from pin P4 on driver IC 82-1may subsequently be output at pin P1 on driver IC 82-1. The output fromdriver IC 82-1 is then received at input pin P4 of driver IC 82-2 (e.g.,the next driver IC in the column).

In other words, the output of each driver IC is provided as the input tothe next driver IC in the chain (e.g., in the column in FIG. 9). Thismeans that output pin P1 of each driver IC is electrically connected toinput pin P4 of an adjacent driver IC. Information provided via signalline 108-1 may therefore be propagated through the driver ICs in a givencolumn. In one example, signal line 108-1 is a digital signal line thatis configured to provide initialization information (e.g., addressinformation) to the driver ICs. The initialization information isprovided to driver IC 82-1 by signal line 108-1. Driver IC 82-1 thenpasses the initialization information to the next driver IC (82-2),which passes the initialization information to the next driver IC, etc.

Each column of driver ICs may have a corresponding digital signal linefor providing initialization information to input pin P4 of at least onedriver IC. As shown in FIG. 9, a second column of driver ICs may have acorresponding digital signal line 108-2 that provides information to pinP4 of driver IC 82-3. Driver IC 82-3 then passes the initializationinformation to the next driver IC (82-4), which passes theinitialization information to the next driver IC, etc.

Each driver IC also includes a pin P3 that is coupled to a respectivesignal line. For example, pin P3 of driver ICs 82-1 and 82-2 are coupledto signal line 104-1. Pin P3 of driver ICs 82-3 and 82-4 are coupled tosignal line 104-2. In other words, each column of driver ICs may have acorresponding signal line that is used to provide information to pin P3of the driver ICs. In this example, signal line 104-1 may be used toprovide LED brightness values to the driver ICs. For example, signalline 104-1 indicates to driver IC 82-1 to update the brightness of itscorresponding LED group 102 to a first given magnitude, indicates todriver IC 82-2 to update the brightness of its corresponding LED group102 to a second given magnitude, etc. Signal line 104-2 indicates todriver IC 82-3 to update the brightness of its corresponding LED group102 to a third given magnitude, indicates to driver IC 82-4 to updatethe brightness of its corresponding LED group 102 to a fourth givenmagnitude, etc.

Signal lines 104 and 108 may be digital signal lines that are used toconvey digital signals. The signal lines may be used to convey data,instructions, or any other desired information. The signal lines maytherefore sometimes be referred to as control lines, data lines, etc.Multiple signal lines may be part of a single bus 110. FIG. 9 shows anexample where signal lines 104-1, 104-2, 108-1, and 108-2 are part ofbus 110.

The LED array may include a plurality of busses, each of which providessignals to a corresponding subset of driver IC columns. In other words,the LED array may have a given number of busses (x). Each of thosebusses may provide one or more signals to a given number of driver ICcolumns (y). Each driver IC column may have a given number of driver ICs(z). Any desired values may be used for x, y, and z. In one illustrativeexample, there may be 24 busses, 2 driver IC columns per bus, and 27driver ICs per column. This example is merely illustrative. In general,the LED array may include any desired number of busses (e.g., 1, 2, morethan 2, more than 5, more than 10, more than 20, more than 30, more than50, more than 100, more than 500, less than 100, less than 40, less than30, less than 20, between 20 and 30, between 20 and 25, between 15 and50, etc.). The LED array may include any desired number of driver ICcolumns per bus (e.g., 1, 2, 3, 4, more than 4, more than 8, less than10, less than 5, between 1 and 4, etc.). The LED array may include anydesired number of driver ICs per driver IC column (e.g., more than 5,more than 10, more than 20, more than 30, more than 50, more than 100,more than 500, less than 100, less than 40, less than 30, less than 20,between 20 and 30, between 25 and 30, between 20 and 50, etc.). Bussesmay also provide signals for partial columns of driver ICs in anarrangement where only part of one or more columns are chained together.

Pin P2 in each driver IC may be coupled to ground (e.g., a ground powersupply voltage). Therefore, each driver IC in FIG. 9 sinks currentthrough its LED group 102 to ground. In the example of FIG. 9, eachdriver IC is coupled between the cathode of the last LED in the chainand ground. This example is merely illustrative. In an alternativearrangement, each driver IC could be coupled between the anode of thefirst LED in the chain and the positive power supply line 106.

The signal lines in FIG. 9 (e.g., 104-1, 104-2, 108-1, 108-2, and 106)may be coupled to a connection area 104 of the LED array. Connectionarea 105 may be, for example, a connector that is coupled to acontroller that is off of the LED substrate. This example is merelyillustrative. In general, any desired connection scheme may be used toprovide desired signals on the signal lines.

Each driver IC may have a length 212 and a width 214. Reducing thecomplexity of the driver IC (e.g., by only having four pins, having eachdriver IC only control 1 LED group, etc.) may allow for the length andwidth of the driver IC to be reduced. The length and width of the driverIC may be any desired respective distances (e.g., less than 0.5millimeters, less than 1.0 millimeters, less than 0.4 millimeters, lessthan 0.3 millimeters, less than 0.2 millimeters, greater than 0.1millimeter, greater than 0.2 millimeters, greater than 0.3 millimeters,between 0.2 and 0.5 millimeters, between 0.30 and 0.35 millimeters,etc.). In one illustrative example, both the length 212 and width 214may be less than 0.5 millimeters. Both the length 212 and width 214 maybe between 0.30 and 0.35 millimeters.

During operation of LED array 36, the driver ICs may be operable in anaddressing phase (sometimes referred to as an initialization phase).During the addressing phase, signal lines 108 assign addresses (e.g.,from an external controller) to the driver ICs. The addresses may bepropagated through driver ICs within a given column. In other words,during the addressing phase each driver IC (except for the last driverIC in the chain) provides an output on pin P1 that is received by anadjacent driver ICs pin P4 (e.g., through a digital signal line that iscoupled between the pins). In some embodiments, the same packet ofinformation may be passed through the driver ICs. In other embodiments,the packet may be modified by a given driver IC before being passed tothe next driver IC in the chain.

During the initialization phase, brightness values may be provided tothe driver ICs using signal lines 104. The brightness values may includea plurality of brightness values, with each brightness valuecorresponding to a respective LED zone 102. The driver IC may receive apacket with brightness values, parse the packet to determine itscorresponding brightness value, and update its target LED brightness tobe equal to the newly received brightness value. The driver IC mayselect the appropriate brightness value out of multiple brightnessvalues within the packet based on the assigned address received viasignal path 108. A single packet with the brightness values may beprovided to the entire LED pixel array, different packets may beprovided on each bus, or different packets may be provided to eachdisplay IC column. Having more unique packets may reduce the amount ofdata that needs to be included in each packet.

After the initialization is complete, the driver IC may switch from theinitialization mode to a normal mode (sometimes referred to as a displaymode). During the normal mode, each driver IC controls its associatedLED zone 102 to emit light with the brightness that was received via thebrightness data at pin P3. To control the brightness of the LED zone102, the display IC sinks a given amount of current to ground at pin P2.The display IC may include, for example, a drive transistor thatcontrols the amount of current that is allowed to pass through the LEDsin zone 102 and therefore controls the brightness of the LEDs.

There are numerous control schemes that may be used to operate the LEDarray of FIG. 9. In one embodiment, a timing controller (TCON) may beused to control the LED array. FIG. 10 is a schematic diagram of anillustrative electronic device with a timing controller that controlsthe LED array 36 of backlight 42. As shown in FIG. 10, electronic device10 may include a timing controller 122 (TCON) on a substrate such ascircuit board 120. Circuit board 120 may be a flexible printed circuitboard or a rigid printed circuit board. Timing controller 122 mayreceive information from a graphics processing unit 132 (GPU) on mainlogic board 130. Main logic board 130 may be a rigid printed circuitboard in one example. GPU 132 may provide data for display 14 to thetiming controller 122. Timing controller 122 controls the pixel array 24and LED array 36 (of the backlight) to display the data.

To control the pixel array 24, the timing controller 122 may use displaydriver integrated circuits 128. The display driver integrated circuits128 (similar to display driver integrated circuits 62A/62B in FIG. 3)may be configured to adjust the liquid crystal display pixels of pixelarray 24 on a per-pixel basis. The pixels may be adjusted to passdifferent amounts of light to achieve a desired per-pixel transparencyand corresponding brightness. Each display driver integrated circuit 128may control a corresponding subset of the pixels in pixel array 24.

Each display driver integrated circuit may be positioned on a respectiveflexible printed circuit 126, as shown in FIG. 10. There may be one ormore optional daughter boards 124 coupled between the flexible printedcircuits 126 and circuit board 120. In general, the depiction of printedcircuits 126 and 124 in FIG. 10 is merely illustrative. Any desiredconnection scheme (e.g., with any desired number of intervening circuitboards, connectors, signal lines, etc.) may be used to couple pixelarray 24 to display driver integrated circuits 128. Similarly, anydesired connection scheme (e.g., with any desired number of interveningcircuit boards, connectors, signal lines, etc.) may be used to couplethe display driver integrated circuits 128 to timing controller 122.

Timing controller 122 may control LED array 36 in unison with pixelarray 24. For example, for a given frame of image data, the timingcontroller 122 may send pixel values to display driver ICs 128 for pixelarray 24 and may send LED brightness values to LED driver ICs 82 for LEDzones 102. In FIG. 10, timing controller 122 sends signals directly todriver ICs 82 on substrate 84. A connecting structure (e.g., a flexibleprinted circuit) 136 may be coupled between circuit board 120 and LEDsubstrate 84. The connecting structure may pass signals from the timingcontroller 122 to the drivers 82 (and optionally from the drivers 82back to the timing controller 122).

FIG. 10 also shows how main logic board 130 may include a boostconverter 134 that is configured to provide a power supply voltage(e.g., VLED) to LED array 36. The various printed circuits shown in FIG.10 may be electrically connected using solder, signal paths, vias, pins,etc.

The number of signal lines between timing controller 122 and LED array36 may be proportional to the number of driver ICs included in the LEDarray. As the size and density of the LED array increases, the number ofdriver ICs included may increase. Increasing the number of driver ICsincreases the number of signal paths required. Routing a high number ofsignal paths between the timing controller 122 and driver ICs may bechallenging due to the limited space available to include all of thedesired signal paths. When the number of LED driver ICs 82 issufficiently small, the timing controller 122 may still send signalsdirectly to the driver ICs. However, as the number of LED driver ICsincreases, it may become preferred to provide a dedicated backlightcontroller for controlling the LED driver ICs 82.

FIG. 11 is a schematic diagram of an illustrative electronic device witha backlight controller that controls the LED array 36 of backlight 42.The arrangement of FIG. 11 is the same as previously shown in FIG. 10,except for the presence of backlight controller 138 (BCON) betweentiming controller 122 and LED driver ICs 82. The backlight controller138 may be mounted on connecting structure 136 (e.g., a flexible printedcircuit) and may receive signals from timing controller 122. Based onthe signals from timing controller 122, backlight controller 138provides signals to the driver ICs 82.

The presence of backlight controller 138 may allow for a reduction ofthe number of signal paths between the timing controller and the LEDdriver ICs 82. Between timing controller 122 and backlight controller138, the number of signal paths may be reduced. Backlight controller 138may, based on the signals from the TCON, provide the full complement ofsignals to driver ICs 82. However, the full signal path routing is onlyrequired in a smaller area (between the BCON and the driver ICs). Thismay mitigate routing and fan-out issues between the LED array 36 andtiming controller 122.

The arrangements of FIGS. 10 and 11 are merely illustrative. In general,the components of LED array 36, pixel array 24, and the correspondingcontrol circuitry (such as BCON 138, TCON 122, GPU 132, boost converter134) may be arranged on any desired number and type of substrates in anydesired combination. The components may be electrically connected usingany combination of solder, signal paths, vias, pins, etc.

In the example of FIG. 9, each LED driver integrated circuit has fourpins. Minimizing the number of pins in each driver IC may advantageouslyminimize routing on the LED substrate 84. Fewer pins in the driver ICsmay also allow the driver ICs to be less complex, and therefore smallerand less expensive to manufacture. However, if desired, the number ofpins in each driver IC may be increased for added functionality. FIG. 12is an example of an LED array where each driver IC 82 has six pins (P1,P2, P3, P4, P5, and P6). Similar to as shown in FIG. 9, each driver ICcontrols the brightness of an associated LED zone 102.

The function of pins P1-P4 may be similar in FIG. 12 as in FIG. 9. PinP1 may again serve as an output pin for each driver IC 82. Thelight-emitting diodes of zone 102 are coupled between power supply line106 and pin P1 of the driver IC. The output pin P1 is also coupled tothe input pin P4 of the next driver IC in the chain, as discussed inconnection with FIG. 9. Pin P2 may be coupled to ground. Pin P4 mayreceive identification information (e.g., addressing information) fromcontroller 138 (e.g., via signal line 108). P3 may receive brightnessvalues similar to as discussed in connection with FIG. 9. The example inFIG. 12 of the controller being backlight controller 138 is merelyillustrative. As shown in FIG. 10, the driver ICs 82 may instead bedirectly controlled by timing controller 122 if desired.

In FIG. 12, the driver ICs also have a pin P5 that is coupled to abi-directional data signal line. The bi-directional data signal line 142may be used for providing control signals or data from controller 138 tothe driver ICs. Alternatively, the bi-directional data signal line 142may be used to convey feedback information from the driver ICs tocontroller 138. For example, the driver ICs may send diagnosticinformation to the controller such as a flag indicating the presence ofa short-circuit, a status of whether or not the driver IC is receivingsufficient voltage, etc. In some embodiments, controller 138 may conveybrightness values to the driver ICs via bus 142 (e.g., the brightnessvalues may be provided to pin P5 instead of P3). In this type ofarrangement, the P3 pin may optionally be omitted or may be used toreceive a different type of signal.

Controller 138 may control the direction of transfer on bus 142.Controller 138 may control the direction of signal transfer using, forexample, a switch 146 that is coupled between the bus (and a resistor148) and a bias voltage supply terminal. Controller 138 may control thestate of the switch 146 to control the direction of signal transfer onbus 142.

In FIG. 12, the driver ICs also have a pin P6 that is coupled to asignal line 144. Signal line 144 may be a digital signal line that isused to provide clock signals to the LED driver ICs. The clock signalsmay be used by controller 138 to control the timing of operation of thedriver ICs.

The driver ICs of FIG. 9 and FIG. 12 each have one output pin forcontrolling LEDs (pin P1). This example is merely illustrative. Ifdesired, the driver IC may include additional output pins to allowindependent control of multiple LED zones. FIG. 13 is a schematicdiagram of a driver IC with multiple output pins for control of multipleLED zones. As shown, LED driver IC 82 includes 9 pins (P1, P2, P3, P4,P5, P6, P7, P8, and P9). Pins P1, P2, P3, P4, P5, and P6 may have thesame functions as discussed in connection with FIGS. 9 and 12. Pins P7,P8, and P9 may serve as additional output pins for control of additionalLED zones. For example, output pin P1 may be used to control LED zone102-1 (e.g., current is passed through the LEDs in zone 102-1 to groundthrough output pin P1 and ground pin P2). Additional output pin P7 maybe used to control LED zone 102-2 (e.g., current is passed through theLEDs in zone 102-2 to ground through output pin P7 and ground pin P2).Additional output pin P8 may be used to control LED zone 102-3 (e.g.,current is passed through the LEDs in zone 102-3 to ground throughoutput pin P8 and ground pin P2). Additional output pin P9 may be usedto control LED zone 102-4 (e.g., current is passed through the LEDs inzone 102-4 to ground through output pin P9 and ground pin P2).

By including the additional output pins, a single LED driver IC isenabled to control multiple LED zones to have different brightnessvalues. For example, in FIG. 13, LED zones 102-1, 102-2, 102-3, and102-4 may all have unique brightness magnitudes, as controlled by thedriver IC 82. Enabling multi-zone control in this way may reduce thenumber of total driver ICs required in the LED array. However, thecomplexity and size of each individual driver IC will increase. Thenumber of pins selected for the LED driver ICs in a display maytherefore depend on the particular design constraints for that display.

The example in FIG. 9 and FIG. 12 of each LED zone including 9 LEDs ismerely illustrative. In general, each LED zone may include any desirednumber of LEDs (e.g., a single light-emitting diode, a pair oflight-emitting diodes, 2-20 light-emitting diodes, at least 2light-emitting diodes, at least 4 light-emitting diodes, at least 8light-emitting diodes, fewer than 5 light-emitting diodes, between 4 and12 light-emitting diodes, between 8 and 12 light-emitting diodes,between 8 and 10 light-emitting diodes, 9 light-emitting diodes, orother desired number of light-emitting diodes). Regardless of the numberof LEDs in the LED zone, the LEDs may be connected in series as shownand discussed in connection with FIG. 9 and FIG. 12.

FIG. 14 is a cross-sectional side view showing how light-emitting diodesand driver ICs may be mounted to the upper surface of the substrate. Asshown, LED array 36 includes a substrate 84 (e.g., formed from glass).Substrate 84 may sometimes be referred to as glass substrate 84 or glasslayer 84. Circuitry layers 150 (sometimes referred to as thin-filmcircuitry may be formed on the glass layer 84. Circuitry layers 150 mayinclude one or more conductive layers that are deposited on glass layer84. The circuitry layers 150 may be patterned to form traces that followdesired paths (e.g., to form signal lines as in FIG. 9). In one example,circuitry layers 150 includes first and second conductive layers with anintervening insulating layer. Glass layer 84 and circuitry layers 150may sometimes collectively be referred to as thin-film layer 152,thin-film glass 152, thin-film circuitry layer 152, thin-film circuitryglass 152, glass substrate 152, LED substrate 152, glass LED substrate152, etc. In other words, the term glass substrate may be used to referto both the individual layer of glass itself (e.g., glass substrate 84)and the collective combination of a glass layer and conductive layers towhich the LEDs are mounted (e.g., glass substrate 152).

As shown in FIG. 14, circuitry layers 150 may include contact pads 154(sometimes referred to as input-output contacts 154, solder pads 154,etc.). The contact pads may be electrically connected to mountedcomponents by solder 160. As shown in FIG. 14, light-emitting diode 38is mounted on glass substrate 152 and is electrically connected tocontact pads 154 by solder 160. In particular, light-emitting diode 38has input-output contacts 156 (e.g., pins, solder pads, etc.) that areattached to contact pads 154 with solder 160. Driver integrated circuit82 is mounted on glass substrate 152 and is electrically connected tocontact pads 154 by solder 160. In particular, driver IC 82 hasinput-output contacts 158 (e.g., pins, solder pads, etc.) that areattached to contact pads 154 with solder 160. As shown in FIG. 14,driver IC 82 may be mounted to thin-film circuitry glass 152 betweenLEDs 38 in the active area of the display.

In the example of FIG. 14, LEDs 38 and driver ICs 82 may be surfacemount technology (SMT) components. This example is merely illustrative,and other mounting techniques may be used to attach LEDs 38 and driverICs 82 to thin-film circuitry glass 152. One advantage of thearrangement of FIG. 14 is that LEDs 38 and driver ICs 82 may be attachedto the thin-film circuitry glass 152 in a single mounting step. In otherwords, because the LEDs 38 and driver ICs 82 have a similar size and aresoldered to the thin-film circuitry glass in a similar manner, theattachment process for both LEDs 38 and driver ICs 82 may be performedsimultaneously. This is advantageous for reducing the manufacturing costand complexity associated with the LED array.

To improve the efficiency of the backlight, a reflective layer 162 maybe formed on an upper surface of thin-film circuitry glass 152.Reflective layer 162 may be patterned to fill portions of the uppersurface of thin-film circuitry glass 152 not already occupied by LEDs 38and driver ICs 82. Said another way, the LEDs 38 and driver ICs 82 areformed in openings in the reflective layer 162.

The reflective layer 162 may be formed from any desired material. As oneexample, the reflective layer may be formed from a diffusive whitematerial (e.g., a white ink spray or a white tape). This example ismerely illustrative. In general, reflective layer 162 may cause diffusereflection and/or specular reflection. In diffusive reflection, anincident ray of light may be reflected in any direction. In specularreflection, an incident ray of light will be reflected at the same angleit strikes the reflective material. Reflective layer 162 may be formedform a metal coating that causes specular reflection, as anotherexample. Reflective layer 162 may have a high reflectance of the lightemitted by LEDs 38 (e.g., greater than 50%, greater than 70%, greaterthan 80%, greater than 90%, greater than 92%, greater than 94%, greaterthan 96%, greater than 99%, less than 99%, etc.). The reflective layer162 may have any desired thickness (e.g., greater than 1 micron, greaterthan 2 microns, greater than 3 microns, greater than 5 microns, greaterthan 10 microns, greater than 25 microns, less than 3 microns, less than5 microns, less than 10 microns, less than 25 microns, less than 100microns, between 3 and 15 microns, between 1 and 25 microns, etc.).Reflective layer 162 may sometimes be referred to as white overcoatlayer 162.

Thermal considerations may also be taken into account in backlight 42with LED array 36. In particular, the components of LED array 36 (e.g.,the LEDs 38 and driver ICs 82) may generate heat during operation of thedisplay. If care is not taken, the heat generation may adversely affectperformance of the display. The glass substrate 84 may have a lowthermal conductivity. Consequently, the heat generated by the componentsmay not be evenly spread across the array.

To promote heat spreading across the backlight, a thermally conductivelayer 164 may be attached to substrate 84. Thermally conductive layer164 may have a high thermal conductivity and thus more evenly spreadsheat across the backlight. Thermally conductive layer 164 may be formedfrom any desired material. Thermally conductive layer 164 may have athermal conductivity of greater than 100 W/mK, greater than 200 W/mK,greater than 300 W/mK, greater than 400 W/mK, between 100 W/mK and 400W/mK, or another desired thermal conductivity. Examples of materialsthat may be used for forming thermally conductive layer 164 (sometimesreferred to as heat spreading layer 164) include metal (e.g., copper,other metals, or combinations of copper and other metals), carbonnanotubes, graphite, or other materials that exhibit high thermalconductivity. If desired, heat spreading layer 164 may be formed fromtwo or more thermally conductive layers of different types (e.g., alayer of copper attached to a layer of graphite, etc.). Polymer carrierfilms may also be incorporated in layer 164 (e.g., to support a layer ofgraphite). In one illustrative example, heat spreading layer 164includes a layer of graphite interposed between two polymer carrierfilms.

An additional technique for distributing heat from the backlight isshown in FIGS. 15 and 16. FIG. 15 is a top view of an illustrativebacklight 42 with exposed conductive layers. Specifically, reflectivelayer 162 may be etched at the periphery of the backlight to exposeunderlying conductive layer 150-1 in circuitry layers 150. LEDs 38 anddriver ICs 82 may also be formed in recesses in reflective layer 162, asshown in FIG. 14. The exposed portion of conductive layer 150-1 extendsin a ring around reflective layer 162. A conductive layer 150-3 may beseparated from conductive layer 150-1 by insulating layer 150-2.Conductive layer 150-1 and insulating layer 150-2 may be etched at theperiphery of the backlight to expose underlying conductive layer 150-3.The exposed portion of conductive layer 150-3 extends in a ring aroundconductive layer 150-1 and reflective layer 162.

The exposed portions of conductive layers 150-1 and 150-3 may be coupledto heat sinks for additional heat dispersion. As shown in FIG. 15,conductive layer 150-3 is coupled to heat sink 166 and conductive layer150-1 is coupled to heat sink 168. Heat sinks 166 and 168 may be formedfrom any desired material or component (e.g., a component of theelectronic device that serves an additional function such a metalhousing component, a dedicated heat sink with fins, etc.). The heatsinks may be attached to conductive layers 150-1 and 150-3 withthermally conductive paste, in one example. Heat sinks 166 and 168 maysometimes be referred to as heat sink structures. The example of FIG. 15with conductive layers 150-1 and 150-3 connected to discrete heat sinksis merely illustrative. In another example, exposed conductive layers150-1 and 150-3 of thin-film circuitry glass 152 may be coupled to asingle heat sink structure.

FIG. 16 is a cross-sectional side view of the backlight shown in FIG.15. As shown in FIG. 16, conductive layer 150-3 is deposited on an uppersurface of glass layer 84. Insulating layer 150-2 is deposited onconductive layer 150-3. Conductive layer 150-1 is deposited oninsulating layer 150-2. Reflective layer 162 is deposited on conductivelayer 150-1. An exposed portion of conductive layer 150-3 is coupled toheat sink 168 and an exposed portion of conductive layer 150-1 iscoupled to heat sink 166.

In addition to the techniques of FIGS. 14-16 for promoting heatspreading and dispersion, the backlight may include temperature sensorsfor active temperature sensing. As shown in FIG. 17, temperature sensors170 may be distributed across the active area of the backlight onthin-film circuitry glass 152. The temperature sensors may providetemperature data to backlight controller 138 using signal paths 174. Thebacklight controller 138 may provide the temperature data to timingcontroller 122. The temperature data from the temperature sensors acrossthe backlight may allow for a 2D thermal profile to be determined forthe backlight. The 2D thermal profile of temperature across thin-filmcircuitry glass 152 may be used to allow for real time opticalcompensation based on temperature. For example, the performance of theLEDs 38 in backlight 42 and the pixels in pixel array 24 may bedependent upon the operating temperature. Using the operatingtemperature from the 2D thermal profile, the LEDs and pixels may beoperated to exhibit desired brightness values in the real-timetemperature conditions.

Each temperature sensor may be formed using any desired technique. Inone possible arrangement, the temperature sensor 170 may be a four-pointresistive sensor that measures temperature based on changes in theresistance of thin-film traces on thin-film circuitry glass 152. Inother words, the temperature sensor may be formed from metal traces onthe glass substrate (e.g., deposited using physical vapor deposition orother desired techniques).

In the example of FIG. 17, a temperature sensor 170 is formed betweeneach group of four light-emitting diodes. This example is merelyillustrative. In general, the temperature sensors 170 may be distributedacross the LED array in any desired pattern. The temperature sensors maybe distributed with a uniform density across the array or with anon-uniform density across the array. The ratio of LEDs to temperaturesensors may be 4 to 1 (as in FIG. 17) or any other desired ratio (e.g.,1 to 1, 2 to 1, 3 to 1, more than 4 to 1, more than 8 to 1, more than 10to 1, more than 25 to 1, more than 50 to 1, less than 8 to 1, less than10 to 1, less than 25 to 1, less than 50 to 1, less than 100 to 1,between 1 to 1 and 10 to 1, between 2 to 1 and 5 to 1, between 4 to 1and 100 to 1, etc.).

The backlight may also include optical sensors for real time sensing ofLED brightness and/or color. As shown in FIG. 17, optical sensors 172may be distributed across the active area of the backlight on thin-filmcircuitry glass 152. The optical sensors may provide optical data tobacklight controller 138 using signal paths 174. The backlightcontroller 138 may provide the optical data to timing controller 122.The optical data from the optical sensors across the backlight may allowfor a 2D profile of brightness and color to be determined for thebacklight. The 2D optical profile across thin-film circuitry glass 152may be used to allow for real time optical compensation. For example,the operation of the LEDs and pixels may account for the real timeoptical conditions. As one non-limiting example, the optical sensor mayprovide data indicating that a given LED has a brightness that is lowerthan expected. The timing controller may, in response, increase thebrightness of that LED until the target brightness level is reached.Each optical sensor may include any desired components for measuringbrightness levels. The optical sensor may have multiple color channels,different optical sensors may have different color channels, all of theoptical sensors may have the same, single color channel, etc.

In the example of FIG. 17, an optical sensor 172 is formed between eachgroup of four light-emitting diodes. This example is merelyillustrative. In general, the optical sensors 172 may be distributedacross the LED array in any desired pattern. The optical sensors may bedistributed with a uniform density across the array or with anon-uniform density across the array. The ratio of LEDs to opticalsensors may be 4 to 1 (as in FIG. 17) or any other desired ratio (e.g.,1 to 1, 2 to 1, 3 to 1, more than 4 to 1, more than 8 to 1, more than 10to 1, more than 25 to 1, more than 50 to 1, less than 8 to 1, less than10 to 1, less than 25 to 1, less than 50 to 1, less than 100 to 1,between 1 to 1 and 10 to 1, between 2 to 1 and 5 to 1, between 4 to 1and 100 to 1, etc.).

Also, in FIG. 17 there is an equal number of temperature sensors 170 andoptical sensors 172. This example is merely illustrative. In general,the number and position of the temperature and optical sensors may beselected independently. There may therefore be different numbers oftemperature and optical sensors if desired, and the temperature andoptical sensors may be positioned in different patterns across theactive area if desired.

In addition to a reflective layer (162) on the upper surface ofthin-film circuitry glass 152, a reflective layer may be included on thelower surface of thin-film circuitry glass 152. FIG. 18 is across-sectional side view of an illustrative backlight that includes areflective layer attached to a lower surface of the thin-film circuitryglass. As shown, reflective layer 176 is attached to the lower surfaceof thin-film circuitry glass 152. Reflective layer 176 may increase theefficiency of the backlight by reflecting light from light-emittingdiode 38.

As previously discussed in connection with FIG. 14, LED 38 may beattached to input-output contacts 154 in circuitry layer 150 by solder160. LED 38 may emit light in direction 182 (e.g., through pixel array24 toward a viewer). However, LED 38 may also emit light in direction184 (away from the pixel array and viewer). Without reflective layer176, the light emitted in direction 184 may be lost within theelectronic device and fail to reach the viewer. To avoid this decreasein efficiency, reflective layer 176 may be present to redirect the lightback in direction 182 through the pixel array. Reflective layer 176 isattached to the opposite side of glass substrate 84 as the LEDs 38 anddriver ICs 82.

Reflective layer 176 may be formed from any desired material. As oneexample, the reflective layer may be formed from a diffusive whitematerial (e.g., a white ink spray or a white tape). This example ismerely illustrative. Reflective layer 176 may be formed from a metalcoating, as another example. In general, reflective layer 176 may causediffuse reflection and/or specular reflection. Reflective layer 176 mayhave a high reflectance of the light emitted by LEDs 38 (e.g., greaterthan 50%, greater than 70%, greater than 80%, greater than 90%, greaterthan 92%, greater than 94%, greater than 96%, greater than 99%, lessthan 99%, etc.). The reflective layer 176 may have any desired thickness(e.g., greater than 1 micron, greater than 2 microns, greater than 3microns, greater than 5 microns, greater than 10 microns, greater than25 microns, less than 3 microns, less than 5 microns, less than 10microns, less than 25 microns, less than 100 microns, between 3 and 15microns, between 1 and 25 microns, between 1 and 5 microns, etc.).Reflective layer 176 may be a coating or may be a layer of tape.Reflective layer 176 may sometimes be described as reflective coating176 or reflective tape 176.

FIG. 19 is a cross-sectional side view of a backlight that includes botha reflective layer and a thermally conductive layer attached to thelower surface of the glass substrate. As shown in FIG. 19, reflectivelayer 176 may be attached to the lower surface of glass substrate 84.Reflective layer 176 may increase the optical efficiency of thebacklight unit. Additionally, thermally conductive layer 164 (asdiscussed in connection with FIG. 14) is attached to the reflectivelayer, such that the reflective layer 176 is interposed between thelower surface of glass substrate 84 and the thermally conductive layer164. Thermally conductive layer 164 may provide heat spreading benefitsin addition to the efficiency benefits from reflective layer 176.

In yet another example, shown in the cross-sectional side view of FIG.20, a single reflective and thermally conductive layer 178 may beattached to the lower surface of glass substrate 84 (instead of separatereflective and thermally conductive layers 176/164 as in FIG. 19).Reflective and thermally conductive layer 178 may have a highreflectivity to increase the efficiency of the backlight. For example,the reflective and thermally conductive layer 178 may have areflectivity that is greater than 50%, greater than 70%, greater than80%, greater than 90%, greater than 92%, greater than 94%, greater than96%, greater than 99%, less than 99%, etc. Additionally, the reflectiveand thermally conductive layer 178 may have a high thermal conductivityto achieve desired heat spreading properties. Thermally conductive layer178 may have a thermal conductivity of greater than 100 W/mK, greaterthan 200 W/mK, greater than 300 W/mK, greater than 400 W/mK, between 100W/mK and 400 W/mK, or another desired thermal conductivity.

FIG. 21 is a cross-sectional side view of an illustrative backlightshowing how the glass substrate may be formed from white diffusiveglass. Instead of clear glass having a high transparency, a whitediffusive glass 84 W may be used as the substrate for thin-filmcircuitry glass 152. The white diffusive glass 84 W may includedispersed particles 186 (e.g., scattering particles) that achieve thedesired diffusion of light. The reflectance of white diffusive glass 84W may be greater than 20%, greater than 30%, greater than 40%, greaterthan 50%, greater than 70%, greater than 80%, greater than 90%, greaterthan 92%, greater than 94%, greater than 96%, greater than 99%, lessthan 99%, etc.

LED driver integrated circuits 82 are distributed across the active areaof the backlight between LEDs 38. Driver ICs 82 do not emit light andcover the otherwise reflective treatments on thin-film circuitry glass152 (e.g., driver ICs 82 may prevent light from reaching reflectivelayer 162, reflective layer 176, etc.). If care is not taken, the driverICs may be visible (e.g., as a shadow on the display when a purely whiteimage is otherwise desired). To prevent the driver ICs from causingvisible artifacts in the display and to increase the efficiency of thebacklight, the driver ICs may have a reflective upper surface.

FIG. 22 is a cross-sectional side view of an illustrative backlighthaving a driver IC with a reflective upper surface. As shown in FIG. 22,a reflective layer 180 may be formed on the upper surface 188 of driverIC 82. Reflective layer 180 may be formed from any desired material. Asone example, the reflective layer 180 may be formed from a diffusivewhite material (e.g., a white ink spray or a white tape). Reflectivelayer 180 may be a metal coating, as another example. In general,reflective layer 180 may cause diffuse reflection and/or specularreflection. Reflective layer 180 may have a high reflectance (e.g.,greater than 50%, greater than 70%, greater than 80%, greater than 90%,greater than 92%, greater than 94%, greater than 96%, greater than 99%,less than 99%, etc.). The reflective layer 180 may have any desiredthickness (e.g., greater than 1 micron, greater than 2 microns, greaterthan 3 microns, greater than 5 microns, greater than 10 microns, greaterthan 25 microns, less than 3 microns, less than 5 microns, less than 10microns, less than 25 microns, less than 100 microns, between 3 and 15microns, between 1 and 25 microns, etc.).

The example in FIG. 22 of a separate reflective layer 180 being formedover driver IC 82 is merely illustrative. In another illustrativeexample, the upper surface 188 of the IC may itself be polished toincrease the reflectivity of the upper surface. In this type ofarrangement, the reflectance of upper surface 188 may be greater than50%, greater than 70%, greater than 80%, greater than 90%, greater than92%, greater than 94%, greater than 96%, greater than 99%, less than99%, etc. When driver IC 82 does have a separate reflective layer 180,the upper surface of the reflective layer 180 may be considered to bethe upper surface of the driver IC (e.g., because the reflective layereffectively forms the upper surface).

FIG. 23 is a top view of an illustrative LED array showing a possibleLED layout. As shown, the LEDs 38 may be arranged according to a zig-zaggrid (e.g., a non-square-grid), instead of a uniform square grid. In auniform square grid, the LEDs may be arranged in straight columns androws (similar to as shown in FIG. 4, for example). This type ofarrangement, however, may result in visible artifacts such as grid muraduring operation of the display. The visible artifacts may be caused bythe periodicity of the uniform square grid. Therefore, arranging theLEDs in a non-square grid as in FIG. 23 may reduce periodicity andmitigate visible artifacts.

As shown in FIG. 23, the LEDs may be arranged according to zig-zag gridlines 190. The LEDs are arranged in a number of rows (‘R’) and columns(‘C’). Within a given row, the grid line defining the placement of theLEDs may follow a zig-zag pattern (e.g., instead of a straight line, thegrid line has a plurality of segments at angles relative to each other).The grid lines that define rows of LEDs may be referred to as horizontalgrid lines. Similarly, within a given column, the grid line defining theplacement of the LEDs may follow a zig-zag pattern (e.g., instead of astraight line, the grid line has a plurality of segments at anglesrelative to each other). The grid lines that define columns of LEDs maybe referred to as vertical grid lines.

The resulting pattern has horizontal and vertical grid lines thatintersect at different angles relative to each other. For example, atsome points, the grid lines are at a right angle (e.g., angle 194)relative to each other. At other points, however, the grid lines are atan acute angle (e.g., acute angle 192) relative to each other. At otherpoints, the grid lines are at obtuse angles (e.g., obtuse angle 196)relative to each other. Angles 192 and 196 may be supplementary angles.

In FIG. 23, the horizontal and vertical grid lines are in a zig-zagpattern. The LEDs may therefore be referred to as being arranged in anon-square grid or non-rectangular grid (e.g., the grid lines do notform rectangles). The rows of LEDs may therefore be referred to aszig-zag rows, rows following a zig-zag pattern, or non-linear rows.Similarly, the columns of LEDs may therefore be referred to as zig-zagcolumns, columns following a zig-zag pattern, or non-linear columns. Asa result of the zig-zag pattern of FIG. 23, there may be numerousdifferent distances between adjacent LEDs within the display. Forexample, some of the LEDs may be separated by a diagonally opposite LED(e.g., in an immediately adjacent row and immediately adjacent column)by distance D1. Other LEDs may be separated from diagonally oppositeLEDs by distance D2 or D3. D3 may be smaller than D1, which may besmaller than D2. This is in contrast to a square grid, where thedistance between each LED and a diagonally opposite LED is uniformacross the LED array.

The example of zig-zag grid lines in FIG. 23 to mitigate periodicity ismerely illustrative. If desired, the grid lines and LEDs may have ahexagonal arrangement, octagonal arrangement, or any other desiredarrangement. Dithering may also be used to add variance to the positionsof the LEDs across the array.

FIG. 24 is a top view of an illustrative LED array with another possibleLED layout. In FIG. 24, each 3×3 group of LEDs (sometimes referred to asan LED zone 102) has a reduced layout footprint relative to in FIG. 23.Similar to as in FIG. 23, grid lines 190 in FIG. 24 include horizontalzig-zag grid lines and vertical zig-zag grid lines. However, in FIG. 23the LEDs are positioned at the intersection points between thehorizontal and vertical zig-zag grid lines. In FIG. 24, only the centerLED 38E is positioned at the intersection point between the horizontaland vertical zig-zag grid lines. The peripheral LEDs of the group (38P)are moved in direction 198 from the grid line intersection towards thecentral LED 38E.

Arranging the LEDs in this way effectively decreases the surface area ofthe footprint of each LED group. Consequently, the distance 200 betweeneach adjacent LED group is greater (e.g., distance 200 between groups isgreater in FIG. 24 than in FIG. 23). Having smaller LED zones mayimprove backlight performance by mitigating halo effect. Halo effect mayrefer to the phenomenon that occurs when a small area on the display isintended to have a high brightness and be surrounded by a low brightnessregion (e.g., a star in a night sky). Ideally, the low brightness regionwould be controllable totally independently from the high brightnessregion. However, if both the intended high and low brightness regionsoccupy the area of one LED zone, there will be a bright ‘halo’ in theintended low brightness region (because the LED zone is set to a highbrightness for the high brightness region on the display). Reducing thearea of each LED zone may mitigate this halo effect (as there is moreresolution to have the intended backlight brightness levels only inintended areas).

The LED zones of LED array 36 may be optimized to have a target energyprofile. FIG. 25 is a graph of an illustrative energy profile, showingbrightness as a function of position for two LED zones. As shown, thebrightness follows profile 202, with one peak for each corresponding LEDzone. The distance between the peaks for the adjacent LED zones is shownas pitch 206. Another relevant property of the profile is distance 204,which is the width of the peak at a brightness that is half of themaximum brightness of the peak (referred to herein as full width halfmaximum or FW). The ratio of the pitch 206 (P) to the distance 204 (FW)may be a key property of the LED zones in a backlight. P/FW for the LEDzones in backlight 42 (e.g., the zones of FIG. 9, FIG. 12, FIG. 23, FIG.24, etc.) may be less than 1.3, less than 1.2, less than 1.1, greaterthan 1, between 1.05 and 1.2, between 1.05 and 1.15, between 1.01 and1.2, etc. The profile shape depicted in FIG. 25 is merely illustrative.The brightness profile of a given LED zone may follow any desired shape.

To achieve a desired emission profile, the center LED of a given LEDzone may be driven with more current than the peripheral LEDs. FIG. 26is a top view of an illustrative LED array showing how a first LED in afirst zone 102-A may be driven with a different current than the LEDs ina second zone 102-B. Having the center LED (‘A’) be driven with a highercurrent may optimize the emission profile of the 3×3 group of LEDs. Asingle driver IC 82 may be used to drive both zones 102-A and 102-B, ortwo discrete driver ICs may be used to drive the two zones. Although theLEDs in zones 102-A and 102-B are driven with different currents and maytherefore be referred to as different zones, the 3×3 group of LEDs maystill be designed to operate together to achieve a desired emissionprofile. Therefore, the 3×3 group may sill be referred to as a unitaryLED group or LED cell.

In the LED group formed by LED zones 102-A and 102-B, the ratio ofcurrent between the peripheral LEDs (‘B’) and the central LED (‘A’) maybe constant. In other words, the LED group still may have a singletarget brightness value, and the driver IC may apply currents per apredetermined ratio to achieve the target brightness and the optimizedemission profile. The example in FIG. 26 of the central LED having adifferent current (brightness) than the peripheral LEDs is merelyillustrative. In general, any of the LEDs within the group may have aunique brightness to help tune the emission profile as desired.

Herein, the LED array with driver ICs in the active area is described asserving as a backlight for a pixel array (e.g., a liquid crystal pixelarray). It should be noted that, if desired, an arrangement of the typeshown herein may be used to form a stand-alone display (e.g., withoutthe external LCD pixels). The LEDs may form display pixels that arecontrolled by driver ICs in the active area.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An electronic device comprising: a plurality of pixels; and a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises: a substrate having an upper surface; a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes, wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, and wherein each driver integrated circuit is soldered to the upper surface of the substrate.
 2. The electronic device defined in claim 1, wherein the backlight has an active area and wherein each driver integrated circuit is positioned within the active area.
 3. The electronic device defined in claim 1, wherein the array of light-emitting diodes comprises a two-dimensional array of light-emitting diodes and wherein the driver integrated circuits are interspersed amongst the two-dimensional array of light-emitting diodes.
 4. The electronic device defined in claim 1, wherein the backlight further comprises: an additional reflective layer attached to a lower surface of the substrate.
 5. The electronic device defined in claim 4, wherein the backlight further comprises: a thermally conductive layer attached to the additional reflective layer, wherein the additional reflective layer is interposed between the lower surface of the substrate and the thermally conductive layer.
 6. The electronic device defined in claim 4, wherein the additional reflective layer is a reflective and thermally conductive layer having a thermal conductivity that is greater than 100 W/mK and a reflectance that is greater than 80%.
 7. The electronic device defined in claim 1, wherein the substrate comprises a layer of white diffusive glass.
 8. The electronic device defined in claim 1, wherein an upper surface of each driver integrated circuit has a reflectance that is greater than 80%.
 9. The electronic device defined in claim 1, wherein each driver integrated circuit comprises an additional reflective layer that covers a respective top surface of that driver integrated circuit.
 10. An electronic device comprising: a plurality of pixels; and a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises: a substrate having an upper surface; a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes, wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, wherein the array of light-emitting diodes is a two-dimensional array of light-emitting diodes that is arranged in a two-dimensional array of respective cells, wherein each cell includes multiple light-emitting diodes, and wherein a spacing between adjacent light-emitting diodes within a given cell is smaller than a spacing between adjacent cells.
 11. The electronic device defined in claim 1, wherein the array of light-emitting diodes is a two-dimensional array of light-emitting diodes that is arranged in plurality of zig-zag columns and a plurality of zig-zag rows.
 12. An electronic device comprising: a plurality of pixels; and a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises: a substrate; a two-dimensional array of light-emitting diodes mounted on the substrate; driver integrated circuits mounted on the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the two-dimensional array of light-emitting diodes and has a respective top surface; and a plurality of reflective layers, wherein each reflective layer is formed on the top surface of a respective driver integrated circuit.
 13. The electronic device defined in claim 12, wherein the driver integrated circuits are interspersed amongst the two-dimensional array of light-emitting diodes.
 14. The electronic device defined in claim 12, wherein each driver integrated circuit is soldered to the substrate.
 15. The electronic device defined in claim 12, wherein each driver integrated circuit is a surface mount technology component.
 16. The electronic device defined in claim 12, further comprising: a reflective layer that is formed separately from the plurality of reflective layers, wherein the reflective layer is formed on an upper surface of the substrate.
 17. An electronic device comprising: a plurality of pixels; and a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises: a substrate having an upper surface; a reflective layer formed on the upper surface of the substrate, wherein the reflective layer has a plurality of openings; an array of light-emitting diodes mounted on the upper surface of the substrate, wherein the array of light-emitting diodes is overlapped by the plurality of pixels; and driver integrated circuits mounted on the upper surface of the substrate, wherein each driver integrated circuit controls at least one light-emitting diode of the array of light-emitting diodes and wherein each light-emitting diode and each driver integrated circuit is positioned within a respective opening of the plurality of openings, wherein each driver integrated circuit is a surface mount technology component. 